Remote-Controlled Termination Beverage Antenna

The RCT Beverage antenna

Beverage front-to-back ratio

As discussed in my forth-coming article "Beverage Antenna
Termination: Why Bother?", the front to back ratio of the Beverage
depends critically on the termination impedance. Figure 1 below shows
the variation in front-to-back ratio versus termination resistance
for a 279 meter Beverage at 1.0 MHz mounted 2 meters above
moderately-poor ground at a wave-angle of 20.7 degrees. In the
example, the terminator must be between about 513 and 654 ohms, or
about 10%, to achieve a front-to-back ratio of 25 dB or more.

Figure 1. F/B ratio versus termination resistance.

It isn't possible to predict the exact value for the termination
resistor that maximizes the front-to-back ratio. The characteristic
impedance of the antenna varies with the height above ground, ground
conductivity, permeability of the earth, and frequency. The effective
height above ground is difficult to determine since RF penetrates the
earth to some extent and thus the effective height is greater than
the height above the surface of the ground. The ground conductivity
depends on the composition of the soil and its moisture content. This
varies both seasonally and with the weather. According to
measurements reported in Belrose, Litva, Moss, and Stevens (Ref. 1),
the characteristic impedance of a Beverage varies approximately 20%
over the frequency range of 2 MHz to 12 MHz.

Ideally you should experimentally adjust the termination
resistance for the deepest null to the rear. This is easier said than
done since the receiver is typically 1000 feet or more from the
terminator. One either needs a good pair of running shoes or a
partner and a pair of walkie-talkies to adjust a potentiometer at the
far end of a Beverage.

The Remote Controlled Termination (RCT) Beverage antenna

I've developed a method to remotely-control a termination resistor
located at the far end of the antenna. I use a cadmium sulfide (CdS)
photocell as the Beverage termination resistance. The brighter the
light on the photocell, the lower its resistance. In my system a 12
volt incandescent lamp illuminates the CdS photocell. I control the
resistance of the photocell by adjusting the voltage on the lamp with
a potentiometer. I use #22 AWG stranded twisted-pair wire both as the
antenna and to feed the control voltage to the lamp. The voltage
across the twisted-pair drives the incandescent lamp at the
terminator. Both wires in the twisted-pair are AC-coupled to the
photocell, which is connected to the ground system.

Cadmium sulfide photocells are good RF resistors. They are fairly
linear so they do not produce much intermodulation distortion. They
have low parasitic capacitance and inductance. For the cell that I
use, at 466 ohms DC resistance, I measured the impedance at 10 MHz as
435 - j110 ohms using a General Radio 916A RF bridge. This is
equivalent to a 463 ohm resistor in parallel with a 8.7 pF capacitor.

Figure 2. typical resistance vs. voltage curve for
VTL3A27.

I use a opto-isolator containing the CdS photocell and
incandescent lamp encapsulated in an epoxy package. EG&G Vactec
manufactures them and sells them under the trade name "Vactrols". I
use the type VTL3A27 "Vactrol" analog opto-isolator. The desirable
characteristics for the CdS cell are a low on-resistance, a low
light-history memory, a low temperature coefficient, and a shallow
resistance versus voltage characteristic curve in the region around
500 ohms and below. The maximum on-resistance needs to be below a few
hundred ohms since the resistance of the ground system is in series
with the terminator, and the necessary termination resistance is
somewhere around 400 to 500 ohms. A low light-history memory and a
low temperature coefficient are desirable to minimize drift in the
termination resistance setting. The shallow resistance versus voltage
characteristic keeps the control setting from being too sensitive.
The VTL3A27 meets these requirements best, but is not available
through distributors like Allied Electronics. I bought mine in
quantity from EG&G Vactec. Figure 2 shows the typical resistance
versus voltage characteristic curve for the VTL3A27.

Figure 3. typical resistance vs. voltage curve for
VTL3A47.

Allied does stock EG&G/Vactec type VTL3A47. This part has a
lower on-resistance, a higher light-history memory and temperature
coefficient, and a much steeper characteristic curve around 500 ohms.
See figure 3. Mark Connelly has had success with these parts by using
a 10 turn potentiometer to keep the control setting from being too
"fiddly".

Figure 4 illustrates a simple application of the remote
termination scheme. This uses a 9V transistor radio battery or a
12-volt lantern battery and a 1K ohm series potentiometer connected
across the twisted-pair antenna wire to provide the control voltage.
The capacitance between the two wires in the twisted-pair couples
them together for RF. The lamp in the opto-isolator connects between
the two wires in the twisted-pair and the photocell connects from one
wire to the ground system. A good ground is needed, preferably at
least four symmetrically-arranged 30 meter long (about 100 feet)
radials.

Figure 4. simple remote-termination circuit.

Mark Connelly has adapted the remote-termination concept to other
types of antennas. He's experimented with it for the termination of
receive-only rhombic antennas and short phased random-wires. See
Figure 9 in Mark's DCP-2 dual controller/phaser article for the
design of a flexible termination-box that can be configured for
Beverages and random-wires or rhombics and terminated loops.

RCT Beverage description

I've developed a design for a remote-controlled termination
Beverage that is well-suited for DXpedition use. It is designed for
portability since most DXers don't have the space for a permanent
Beverage installation.

The design uses rugged "low-tech" circuitry. The antenna has to
survive the large voltages induced by nearby lightning strikes so I
avoid the use of semiconductors such as FETs or LEDs. The
incandescent lamp and CdS photocell in the terminator are quite
rugged in this respect. Semiconductors are also likely to produce
intermodulation in the presence of strong RF. The CdS photocell is a
quite linear resistance; in my experience it does not cause any
significant intermodulation products.

The RCT Beverage consists of a controller, an impedance-matching
transformer, two 100 foot (30 meter) radials of #24 gauge wire for
the ground system at the transformer, 1000 feet (300 meters) of #22
gauge twisted-pair antenna wire, the remote controlled terminator,
and four 100 foot (30 meter) radials of #24 gauge wire for the ground
system at the terminator. See figure 5.

Figure 5. RCT Beverage components.

The antenna wire is spooled on a 14 inch cord-wheel. It connects
to the terminator through a pair of banana plugs. A pair of
insulation-piercing test clips connect the antenna wire to the
matching transformer. The insulation-piercing test clips allow you to
reel out the optimum length of wire for your frequency of interest.
You simply clip-in to the wire leaving the remainder spooled on the
cord-wheel. To ease the task of measuring the antenna length, I've
had a special-run made of twisted-pair wire marked at 5 meter
intervals.

As discussed in my forth-coming article "Beverage Antenna
Termination: Why Bother?", the front-to-back ratio of a Beverage
varies considerably with the length of the antenna, reaching a local
maxima at intervals of one-half wavelength. If you have a specific
frequency for which you wish to optimize the antenna, adjust the
length of the antenna to a multiple of one-half wavelength at that
frequency, allowing for the velocity factor of the antenna which
varies from 70% to 90% depending on the height above ground (Ref. 2
and 3).

1.0 MHz, 2 meters high, moderately-poor soil, 20.7 wave
angle

Figure 6. F/B ratio
versus antenna length.

Controller

The controller couples a variable DC voltage onto the coax
feedline to the matching transformer. Coupling capacitor C6 blocks
the DC from the receiver input. RF choke L2 blocks the RF signal from
the control circuitry. V5, a Siemens gas-discharge tube, protects the
controller and the receiver against transient voltages.

A #1815 incandescent lamp protects the controller against
short-circuits. It limits the current to a maximum of 200 mA. The
lamp in the opto-isolator takes a maximum of 40 mA so the
current-limiting lamp has little effect unless there is a
short-circuit. Then the lamp serves as a "short" indicator as well as
current limiter.

Figure 7. Schematic of RCT Beverage controller.

R1, a 1K ohm 2-watt linear potentiometer, controls the DC voltage
fed to the lamp. At its maximum resistance 12 mA flows through to the
opto-isolator. This gives a termination resistance of greater than
100K ohms, which is high enough to effectively be an open-circuit.

A 50 milliampere meter serves to detect an open-circuit if the
clips don't pierce the insulation on the antenna wire, if the wire
breaks, or if a moose drags off the antenna. It also allows you to
make a rough estimate of the termination resistance. A 1N4001 diode
connected in reverse across the DC input provides polarity protection
for the meter. If the power source is connected with the wrong
polarity, the diode will conduct and the current limit lamp will turn
on, limiting the reverse voltage across the circuit to about 0.7
volts. This is small enough to prevent damage to the meter.

Matching transformer

The impedance-matching transformer contains a 9:1
unbalanced-to-unbalanced transmission-line transformer. It transforms
the approximate 450 ohm impedance of the Beverage antenna down to 50
ohms to match the coax feedline. Without impedance-matching, there is
a loss of 14 dB between a 450 antenna source and the 50 input of the
receiver. The transformer consists of 5 trifilar tight-wound turns of
#30 AWG Kynar wire-wrap wire on an Amidon Associates FT50-75 ferrite
toroid core. I use Kynar-insulated wire rather than enameled magnet
wire in order to raise the impedance of the trifilar transmission
line. Similarly, the wires are tight-wound in parallel rather than
twisted together in order to maximize the impedance of the trifilar
line. The type-75 core material is high-permeability, so five turns
is sufficient to give more than enough inductance to cover the low
end of the broadcast band. The measured -3 dB point of the entire
controller and matching-transformer system is about 200 kHz at the
bottom and well above 10 MHz at the top.

Figure 8. Schematic of RCT Beverage transformer.

Coupling capacitor C1 prevents the primary winding of the
transformer from shorting the control voltage to ground. Coupling
capacitor C2 couples the twisted pair antenna wires together for RF
but blocks the DC control voltage. The transformer couples the DC
control voltage on the center conductor of the coax feedline to one
of the pair of antenna wires. RF choke L1 provides a DC connection to
the coax shield for the remaining wire.

A pair of gas-discharge tubes (V1 and V2) protect the transformer
from transient voltages.

Terminator

The incandescent lamp in Y1, the VTL3A27 opto-isolator, connects
directly across the pair of antenna wires. Coupling capacitors C3 and
C4 block the DC control voltage from the CdS photocell. The photocell
in Y1 connects directly to the ground system.

Figure 9. Schematic of RCT Beverage terminator.

Two gas-discharge tubes (V3 and V4) protect the opto-isolator from
transient voltages. I used to use NE-2 neon lamps as surge voltage
protectors, but after a few nearby lightning strikes the NE-2's give
up the ghost. The failure mode is interesting in that there is no
visible damage. They appear intact but the firing voltage is very
high, possibly due to sputtering removing the rare earth coating on
the electrodes. I was losing an opto-isolator every week in the
summer until I finally discovered the bad NE-2's. The induced
transient from the lightning strike would usually take out the
incandescent lamp. This shows up immediately on the controller meter
as an open circuit.

I occasionally encountered another failure mode where the
photocell resistance gradually increases. Presumably the transient
vaporizes some of the CdS material and slightly narrows the
resistance track, which slightly increases the on-resistance. Over
time, the minimum on-resistance goes up enough so that the antenna
will no longer null. This can be a real puzzler - I initially
suspected problems with the ground system until I finally found the
bad photocell and NE-2 lamps. I now use heavy-duty gas-discharge
tubes rather than NE 2 neon lamps.

Parts List

Most of the components are readily available from mail-order
suppliers such as Digi-Key, Mouser, and Allied. VTL3A47 Vactrols are
available from Allied. VTL3A27 Vactrols are available for $6.00 each
plus $2.00 shipping and handling per order from:

Assembled RCT Beverage systems are also available from Oak Ridge
Radio.

Figure 10. Parts list for RCT Beverage.

Antenna installation

I use four foot ground rods purchased from Radio Shack as
mechanical supports for the terminator and matching transformer. A
large Mueller battery clip grips the ground rod and the terminator or
matching transformer enclosure plugs onto a banana jack attached to
the battery clip. The ground radials also clip onto the ground rod
using alligator clips. I use two radials on the matching transformer
and four at the terminator. The two on the matching transformer
aren't really necessary. They do increase the received signal
strength somewhat. The four radials at the terminator are absolutely
necessary to get the ground impedance low enough to successfully
terminate the Beverage. More radials are better. Arrange the radials
symmetrically about the ground rod and antenna so that any signal
pickup will cancel out.

Those hardy souls who DXpedition from a tent or cabin using
battery power needn't bother with coax feedlines. For the rest of us,
I recommend placing the matching-transformer end of the Beverage at
least 50 feet (15 meters) away from any power-lines or structures
containing electrical wiring. Most sources of radio-frequency
interference are not very good antennas, so most of the noise pickup
comes from the near-field. The intensity of the near-field diminishes
with the third or fourth power of distance, so moving the antenna a
little further away from local noise sources makes a tremendous
difference in the received RFI noise level.

To avoid degrading the directivity of the Beverage by pickup in
the feedline from the matching transformer to the controller, use
only quality coaxial cable with a 95% or better shield braid
coverage, such as Belden 8259 (a good RG-58 type cable) or RG-6 CATV
cable. In particular avoid Radio Shack RG-58 cable as it has poor
shield coverage and consequently is quite leaky.

I find it very helpful to use two grounds on the coax shield, one
at the matching transformer and one near where the coax cable enters
my shack or DXpedition cabin. The second ground helps prevent RFI
from the house from traveling down the outside of the coax and
coupling into the inside of the coax at the matching transformer.

For similar reasons, I recommend keeping the coax on the ground
(or for permanent installations, buried) rather than suspended off
the ground. The lossy earth absorbs RFI traveling on the outside of
the coax.

Here in New England the forests contain a lot of brush which makes
an excellent support for temporary Beverage wires. For situations
where one can't improvise supports, I recommend procuring a bundle of
hardwood or bamboo garden stakes.

I try to place the wire up about one-and-a-half to two meters off
the ground. Lower is OK, but the increased loss reduces received
signal strength and tends to blunt the nulls.

Gently slope the antenna wire down to ground level at the
terminator and matching transformer rather than running it
vertically. Vertical runs will act as short omnidirectional antennas
and will spoil the directivity of the Beverage. Use about a 1:6
slope; i.e. for a wire 2 meters high, slope the wire down over a
length of about 12 meters or 40 feet. This results in about a 10
degree angle.

If you have a specific frequency for which you wish to optimize
the antenna, adjust the length of wire to a multiple of one-half
wavelength at that frequency, allowing for the velocity factor of the
antenna which varies from 70% to 90% depending on the height above
ground (Ref. 2 and 3).

Antenna operation

In practice it's difficult to achieve null depths greater than 30
dB or so. The AGC range of most receivers is at least this large, so
you won't hear any audible change in signal strength unless you
disable the AGC. I find it easiest to adjust the remote termination
by switching off the AGC, adjusting the RF or IF gain to avoid
overload, and slowly rotating the termination control until I hear
the null. I've tried using the S-meter, but the controlled-carrier
schemes used by many mediumwave stations causes the S-meter to bounce
around with the modulation. This makes it difficult to locate the
deepest null.

The local 50 kW clear-channel station WBZ-1030 is located on a
bearing almost directly behind my tropical-band Beverage aimed at
Papua New Guinea. This Beverage is 175 meters long and about 3 meters
off the ground. For these antenna parameters, the theory predicts a
ground-wave front-to-back ratio of about 22 dB. When the Beverage is
unterminated, I measured WBZ at -29 dBm. When adjusted for maximum
rejection of WBZ, the signal strength drops to -52 dBm, for a
null-depth in this case of 23 dB. This measurement is in fairly good
agreement with the theory.